For contacts in power-engineering switchgear, for instance in power switches as well as in direct current, motor and auxiliary contactors, contact materials of the silver/metal (AgMe) system have proven suitable for a long time. Representatives of this system are, for instance, silver-nickel (AgNi) and silver-iron (Ag--Fe).
Silver-nickel has advantageous contact properties which, together with the test methods for contact materials are described for instance in INT.J Powder Metallurgy and Powder Technology, Vol 12 (1976), pp. 219-228. There is the disadvantage, however, that nickel dust, in particular, has injurious effects on the human organism.
The use of iron in silver-based contact materials has been proposed on various occasions. It has, however, been found that silver-iron does not yet satisfy the requirements stipulated, since, upon the continuous passage of current through such contacts, excessive heating takes place which can be explained by the formation in this system of a cover layer, thus increasing the contact resistance. Furthermore, from Federal Republic of Germany OS 3 813 311 (WO-A-89/10417) a sintered contact material of the aforementioned type is known which, in addition to silver as active component, contains at least iron and/or titanium. Particularly when the iron and the titanium are present in alloyed form and form an intermetallic compound, good results were obtained in switching tests. Nitrides, carbides and/or borides of metals, in particular of titanium, can possibly be present as further active components.
Furthermore, in DE-C-38 16 895 the use of a silver-iron material having 3 to 30 wt.% iron and one or more of the additions manganese, copper, zinc, antimony, bismuth oxide, molybdenum oxide, tungsten oxide and chromium nitride in amounts of a total of 0.05 to 5 wt.%, balance silver, is proposed for electric contacts. Finally, from DE-A-39 11 904 a method of powder metallurgy is known for the manufacture of a semifinished product for electric contacts from a composite material having a base of silver with iron in which 5 to 50 wt.% iron is used as first secondary component and 0 to 5 wt.% of a second secondary component from one or more substances from the group which contains the metals titanium, zirconium, niobium, tantalum, molybdenum, manganese, copper and zinc as well as their oxides and carbides.